1. Field of the Invention
The present invention relates to a projection display device in which a light beam emitted from a light source is modulated by a light valve using a liquid crystal panel or the like, and the modulated light beam is enlarged and projected via a projection optical system.
2. Description of Related Art
A projection display device has been known in which a light beam emitted from an illumination optical system is converted into a modulated beam corresponding to image information by using a liquid crystal light valve, and the modulated beam is enlarged and projected onto a screen (projection plane) via a projection optical system. FIG. 6 shows an optical system in such a projection display device. As this figure shows, a projection display device 1000 comprises an illumination optical system 80 including a light source 81 and a uniform illumination optical system 82, a liquid crystal light valve l00A to be irradiated with light by the illumination optical system 80, and a projection optical system 60A for enlarging and projecting emergent light from the liquid crystal light valve 100A onto a screen 900.
In the projection display device 1000 having such a conventional configuration, the f-numbers F.sub.L and F.sub.P of the illumination optical system 80 and the projection optical system 60A are designed to be nearly equal to each other.
In general, the projection optical system 60A is frequently represented by a single lens element, as shown in FIG. 6, whereas it is normally composed of a plurality of lens elements. This means that a plurality of lens elements can be replaced by a single lens element having a function equivalent to that of those lens elements. It is assumed that the f-number F.sub.P of the projection optical system refers to the f-number of the single lens element thus replaced, that is to say, a value 1.sub.P /d.sub.P that is obtained by dividing a focal distance 1.sub.P of the single lens element by a diameter d.sub.P of the lens.
Furthermore, it is assumed that the f-number F.sub.L of the illumination optical system refers to the f-number of a lens element disposed on the most downstream side of the optical path, that is, closest to the liquid crystal light valve 100A, which is an object to be illuminated, among a plurality of lens elements included in the illumination optical system. When the lens element disposed on the most downstream side of the optical path is a lens array composed of a plurality of small lenses, as shown in FIG. 6, the f-number F.sub.L of the illumination optical system does not refer to the f-number of each small lens, but to a value 1.sub.L /d.sub.L obtained by dividing a distance 1.sub.L from the lens array to an object to be illuminated by the maximum diameter d.sub.L of the lens array (e.g., the diameter in the case of a circular lens array, the length of the diagonal line in the case of a rectangular lens array).
On the other hand, projection display devices have recently been required to display high-definition images. In order to display a high-definition image, it is necessary to increase the number of pixels in a liquid crystal light valve. In a liquid crystal light valve in which pixels are arranged in a matrix, the peripheries of the pixels are shielded from light by a light-shielding layer called a black matrix. Therefore, as the number of pixels in the liquid crystal light valve increases, the region occupied by the black matrix enlarges, and conversely, the area of pixel apertures decreases. That is to say, the amount of light emitted from the liquid crystal light valve decreases as the definition of the liquid crystal light valve increases, and therefore, a projection image produced by the projection display device becomes dark as the definition of an image to be projected increases.
In order to avoid such harmful effects, a liquid crystal light valve has been proposed which is provided with a microlens array composed of a plurality of microlenses so that light is efficiently guided to corresponding pixel apertures by the microlenses.
While incident light can be efficiently guided to the pixel apertures in the liquid crystal light valve provided with the microlens array, emergent light from the liquid crystal light valve, however, is diffused by the lensing action of the microlens array. For this reason, the inclination angle of emergent light from the liquid crystal light valve having microlenses is greater than that of emergent light from a liquid crystal light valve that does not have microlenses.
Therefore, if a liquid crystal light valve having microlenses is incorporated in the conventional projection display device 1000 in which the f-numbers of the illumination optical system 80 and the projection optical system 60A are designed to be nearly equal, there is a fear that a part of emergent light from the liquid crystal light valve will not be drawn into the projection optical system 60A. As a result, light utilization efficiency is lowered, and an image projected on the screen becomes dark.